Automatic volume control in voice frequency circuits



Feb. 25,- 1947. E. T. JONES 2,416,321

AUTOMATIC VOLUME CONTROL IN VOICE FREQUENCY CIRCUIT Filed Deo. 8,A 19433l/VZlE/TOR Y M ATTOR EY Patented Feb. 25, 1947 AUTOMATC VOLUME CONTROLIN VOICE FREQUENCY CIRCUITS Elwyn Tudor J ones, Eltham, London, England,as-

signor, by mesne assignments, to International Standard ElectricCorporation, New York, N. Y., a corporation of Delaware ApplicationDecember 8, 1943, Serial No.' 513,369 I n Great Britain April 9, 1943 8Claims. (Cl. 179-84) This invention relates to circuits for the oper-yation of a signal receiving device by signal currents of a predeterminedfrequency lying within the voice frequency range whilst preventing itsoperation by unwanted currents, also lying within this same range offrequencies, both types of currents being supplied to the device from asingle communication channel.

There are various limiter circuits known in the art which limit bychopping the peaks of the currents under control. The present inventionmakes use of automatic volume control instead of a limiter circuit, andthis former device provides several advantages. Unlike limiter circuitsthe automatic volume control may be adjusted to cause the peaks inunwanted speech currents to depress the average speech currents whichmay reach the signal receiving device to a low value for a considerablelength of time and in addition the same circuit provides a substantiallyconstant level of the desired signalling currents without distorting thewave form thereof. This results in more reliable operation over a widerange of signal input levels because the low harmonic content of suchnon-distorted signal currents reduces filter llosses and makes possiblethe design of simpler and more effective Lguard circuits,

which prevent operation of the receiving device, which may be a relaywhen certain frequencie are present on the communication level. i

According to the present invention a circuit of the kind set forth abovecomprises a variable transducer, a filter passing signal frequencies andexcluding frequencies outside the signal range and a signal receivingdevice connected together in series and a circuit for automaticallyvarying the amplitude of the currents passed by said transducer ininverseproportion to the amplitude of the currents at the input of saidfilter, this inverse proportion being modified by means acting toneutralize the amplitude control when the energy of the input currentsfalls below a predetermined reference level, this reference level beingeither constant, or being made to vary when the signal current ofpredetermined modulation frequency is present upon the communicationchannel.

The nature of the invention will be better understood from the followingdescription taken in conjunction with the accompanying drawings inwhich:

Fig. 1 shows a circuit according to the invention using a variableattenuator;

Fig. 1Al shows a modication Fig. 1; and 'f Fig. 2 shows the circuit ofFig.. 1 with an additional amplitude control.

Referring to the drawings, Fig. 1 shows a circuit for the operation of asignal device X by currents in the'voice frequency range connected ofthe circuit of at the terminals l, 2 to a circuit over which both speechcurrents and signalling currents pass'.v

The circuit comprises a variable attenuator VA, an amplifier A and alter F in series. The attenuator and amplifier together constitute thetransducer. Two diagonal points of a rectifier bridge R are connected tothe secondary of a transformer L3 which forms'part of the filter F, andthe two opposite diagonal points are connected to the signal device X.This deviceA may be of any suitable type, as used in the telecommu- Thefilter comprises inductance's LI, L2, L3 shunted by c-ondensers C I, C2,C3. The

nication art.

loss in the attenuator VA is controlled by acurrent derived from theamplified currents and passing through a rectifier Wl. The automaticvolume control is prevented from acting below a certain minimumamplitude of received vcurrents by an opposing voltage V, acting througha reoti Iier W2, a condenser C4 being inserted to isolate this opposingvoltage from the amplifier A and filter F. A condenser C5 is used tomaintainthe automatic volume control in action over a predeterminedperiod of time, as is usual. y The filter F is designed to passfrequencies intended to operate the signal device X and the tunedcircuits LI, CI and L2, C2 are preferably tuned to frequencies outsidethat range. These circuits thus cause the filter F to present a highimpedance to frequencies outside the signalling range. This provides aneffective and economic guard circuit, since the high impedance of thefilter to frequencies outside the signalling range increases therelative voltage magnication at such frequencies at the lter input andthus causes these frequencies to exercise an accentuated effect inincreasing the loss through'the attenuator. In addition or as analternative to this arrangement of the impedance of lter F, the effectof unwanted frequencies in the automatic volume control circuit may beaccentuated by placingin front of the attenuator an equalising network,EN, which, paradoxical thoughr it may appear, is arranged to causegreater loss to signal frequencies than to unwanted frequencies and inparticular to pass with small loss unwanted frequencies close to thesignal frequency and harmonics of the signal frequency, rangement isshown in Fig. 1A. The operation of the equallzing network is similar tothat of the filter in that the network tends to provide un` Wantedsignals at the output of the amplifier which are greater in amplitudethan the desired signal. Since the automatic volume control circuitoperates on the difference in signal levels, it may be adjusted toprovide attenuation to the unwanted signals so that they are reduced tothe same level'resulting with the nlter alone. When the filter and theequalizing networks are used together their effects are additive,

This ,ar-

q in the same manner Aas in Fig. 1.

' naldevice X.

3 Because the automatic volume control does not chopv the peaks of thesignalling currents it is well suited for use with modulated (includinginterrupted) carrier signallingl currentsI since it provides volumecontrol with-out loss of ercentage modulation` so that reliablefunctioning is possible over a wide lrange of input levels.

At the same time, the circuit may, as shown y age doubling network Riland the resulting rectii'iedcurrent is applied to the control of thevariable attenuator VA in opposition to the normal control circuit. lnthis way all currents will be reduced to aflow value except a pro-perlymodul lated signaler the right carrier frequency. Such asignal willproduce a neutralising current and WilLrise to a high level and willoperate the sig- When current derived from transformer T! is rectified,the resultant direct current voltage will vary asa function of theenergy content of the modulation of the signal. Therefore the referrencelevel below which energy derived via 'C4 can function to effect controlof the variable attenuator` will be raised during the persistence ofsuch modulation currents, due to the increased value ofthe neutralizingcurrent. This causes the attenuator to deliver maximum energy to thesuc-` ceeding elements of the System, thereby operating signal device Xso as to bring about the maximum effective sensitivity thereof. What isclaimed is:

1. A circuit forthe operation of a Areceivingv device by signal currentswithin the voice frequency range whilst preventing its operation byunwanted currents comprisingv a variable trans` ducer, a filter passingsignal frequencies and eX- cluding frequencies outside the signal rangeand a signal receiving device connected together in series and a circuitfor automatically varying the amplitude of the currentsA passed by saidtransducer in inverse proportion to the amplitude of the currents at theinput of said lter, above a predetermined minimum reference level.

2. A circuit as claimed in claim l comprising means for automaticallymaintaining the amplitude of currents passed by said transducer at a'value determined by a peat: of received energy for a period oftime. Y

3. A'circuitas claimed in claim l in whichr said lter presents to said'transducer such an impedance for frequencies outside the signal rangeas to accentuate the effect of the presence of said frequencies inreducing the amplitudeof currents passed by said transducer.

Ll. A circuit as claimed in claim l in whichsaid transducer is precededby an equalising network causing greater loss at signal frequencies thanl at unwanted frequencies.

5. A system for operating a receiving device by signal currents within adesiredY frequency range but'not by currents of frequencies outside isaid range,said system comprising a transducer This frequency,

.Whose transmission efficiency may be varied, a

filter connected to receive signals from said trans` ducer and to passonly the desired frequency range, said receiving device being connectedto receive signals from said filter, and an automatic `volume controlcircuit responsive to the amplitude of signals at the input of saidfilter for varying the transmission emciency of said transducer ininverse relation to the amplitude of said current, above a predeterminedminimum reference level.

6. A system for operating `aireceiving device by signal currents withina desired frequency range but not by currents of frequencies outsidesaid Y range, said system comprising a transducer having a variabletransmission efficiency, a filter connected to receive energy from theoutput of said` transducer and to pass only the desired frequency range,said receiving devicebeing connectedA to receive signals from the outputof said filter, and an automatic volume control circuit adapted toreceive energy from a position between said transducer and said filter,said automatic volume control circuit including elements for developinga voltage responsive to the energy impressed on said volume controlcircuit and varying the transmission eiciency ofv said transducer ininverse relation thereto, above a predetermined minimum reference level.

"7. A system for operating a receiving device by signal currents withina desired frequency range but not by currents of frequencies outsidesaid range, said system comprising a transducer having a variabletransmission efficiency, a lter connected to receive energy from theoutput of said transducer and to pass only the desired frequency range,said receiving device being connected to receive signals dependent'onthe output of said filter, -an automatic volume control circuitresponsive to the amplitude of energy at a point between saidtransducerand Said lter, and a second volume control circuit responsiveto energy at said receiving device, said second volume control circuitbeing' connected in opposition with the first mentioned volume contr-olcircuit, and a connection from said volume control circuits to thetransducer for varying the transmission emciency of said transducer ininverse relation with the energy derived from said first volume controlcircuit, above a level determined bythe energy derived from said secondvolume control current. `8. A system according to claim 7 in which thesecond volume control circuit comprises circuit elements for separatingfrom the signal at said receiving device, Currents having the frequencyof signal modulation, rectifying said currents and ELVVYN TUDOR JREFERENCES CITED The following Yreferences are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name vDate 2,334,468 Adams n v Nov. 16,1943 2,084,135 u f Ford June 1 5, '1937-

